506 Mr. E. S. Johoimott on the Thickness of 



exposed to heat or to the atmosphere of the room. (The 

 films will be said to be " exposed " when the casing is unsealed 

 and the atmosphere about the films is exposed to that of the 

 room at crevices along the casing M (fig. 2).) Solutions of 

 oleate of soda or potash, without any salt or glycerine added, 

 show the greatest and most sudden changes in thickness. It 

 was observed that these sudden changes were always accom- 

 panied by the formation of a second black film within the 

 first. When the case was open or only partially sealed, this 

 second black was seen to form first on the more exposed films 

 in the case, and on the side of the frames toward a distant 

 source of heat. A group of round spots would appear, at 

 least two millimetres from the frames, and would expand and 

 multiply until the whole upper part of the films was covered 

 with the second black. This was also the case with films 

 tightly sealed if the casing was warmed with a flame. With 

 exposed or heated films the second black would soon appear on 

 all the frames, after it had formed on the most exposed. 

 Many times it would form by the expansion of a single spot, 

 appearing as though the spot were a region of slightly 

 smaller surface-tension in the first black film. 



The slightest occurrence of the second black film was dis- 

 tinctly seen by arranging a gas-jet at L (fig. 1) , and observing 

 the reflected light. The spots would occur first on the upper 

 corner nearest the light, unless the side of the case opposite 

 the light was unsealed, while that toward it was sealed ; then 

 they would form first on the more exposed side. Oftentimes 

 a thick silvery film would form on the frames, as a boundary 

 to the second black film. 



This could usually be produced by warming an exposed 

 film. 



The second black on a liquid film was first observed by 

 Newton*. Remold and Riickerf on one occasion succeeded 

 in getting the two black films successively between their 

 needles. The resistance of the two films indicated thicknesses 

 of 200 /ul/jl and 110 fifi respectively. These numbers are evi- 

 dently much too large, the error being due, as their experi- 

 ments show, to a difference in the specific resistance of the 

 film and that of the solution. 



Again, the second black film will not appear when the 

 films have been sealed in for any length of time, and the first 

 black film may remain at a high thickness, particularly if 

 the temperature is low and not increasing. In all cases, 

 however, the limiting thickness of the first black film, before 



* Lord Kelvin, ' Constitution of Matter,' p. 177. 

 t Phil. Trans, (a) 1893, p. 512. 



